The present invention relates to a tubular jacket for an absorber tube of a solar energy collector, especially a parabolic trough collector. The invention also relates to a parabolic trough collector for solar energy and a receiver tube for it.
A known parabolic trough collector comprises a single axis parabolic mirror and a receiver tube, which is arranged at the focus of the parabolic mirror. The mirror usually has a width of from 5 to 6 m. The receiver tube comprises a preferably radiation selective inner tube section, which is also called the absorber tube, and an outer tubular jacket made of glass for insulation. Mirror and receiver tube are directed toward the sun, so that the solar radiation always is directed normal to the aperture plane and ideally the radiation falling on the mirror is guided to the receiver tube.
Focusing errors and thus geometrically dependent optical losses occur in parabolic trough collectors due to various factors. For example, the mirror elements have a certain total shape tolerance or also waviness, which leads to focusing errors. The positioning of the mirror elements during assembly is only possible within certain tolerances. Also self-deformation, manufacturing and assembly tolerances of the steel structure, on which the parabolic trough collector is built, must be considered. Last, but not least, wind occurring in the vicinity of the parabolic trough collector leads to deformation of the entire structure and thus to focusing errors.
Currently attempts have been made to minimize optical losses by defocusing with the help of secondary concentrators mounted on the receiver tube. Already there has been an experimental use of a secondary planar reflector. An additional arrangement of a parabolic mirror with a secondary concentrator in the form of a metallic reflector has been described in WO 97/00408. A zig-zag-shaped metal sheet has been used as a secondary concentrator according to H. Price, et al, Journal of Solar Energy Engineering, Volume 124, p. 109-125 (2002).
When a highly reflective material, e.g. a polished metal sheet, is used for the secondary concentrator, it is important to put it in the tubular jacket in a vacuum, in order to protect it from dirt and aging. The secondary concentrator can be mounted either on the tubular jacket or on the absorber tube. The absorber tube is shaded by mounting the secondary concentrator above the absorber tube on the side facing away from the mirror. When the secondary concentrator is wider than the absorber tube, also a part of the mirror is shaded. If the secondary concentrator is attached to the tubular jacket, a part of the radiation, which falls on the side of the secondary concentrator facing away from the mirror, is lost, since the tubular jacket and the absorber tube are thermally decoupled. It is possible to use a portion of this radiation when the secondary concentrator is attached to the absorber tube and is made absorbing on the side facing away from the mirror. Because of that feature more radiation can be utilized. At the same time however the increase in the absorber surface area increases the thermal losses.
The increase of the interceptor factor (the fraction of the radiation, which falls on the absorber tube), which is achieved by the secondary concentrator, is necessarily accompanied with radiation losses on account of the above-mentioned disadvantages. No significant improvement of the interceptor factor may therefore be achieved in total.